Photoaffinity labeling of skeletal myosin with 2-azidoadenosine triphosphate

The purine binding site of ATP on skeletal muscle myosin has been photoaffinity labeled with 2-azidoadenosine diphosphate (2-N3ADP). 2-N3ADP was stably trapped at the active site (t1/2 approximately 5 days, 0 degree C) by complexation of the two heavy chain reactive thiols (Cys-697 and Cys-707) with Co(III)phenanthroline. Photoincorporation occurred only in the 23-kDa NH2-terminal tryptic fragment of the heavy chain. Extensive serial digestion of photolabeled subfragment 1 of myosin by trypsin and subtilisin yielded a series of labeled peptides which were purified by HPLC. Sequence and radiolabeling analysis of eight photolabeled peptides all indicated that tryptophan-130 was the only labeled residue. This site of labeling confirms earlier photolabeling studies with the non-nucleotide ADP analogue, 2[(4-azido-2-nitrophenyl)-amino]ethyl diphosphate (NANDP), which also labeled Trp-130 [Okamoto, Y., & Yount, R. G. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 1575-1579]. Comparison of the structures of 2-N3ADP and NANDP indicate that their azido groups can be superimposed if both analogues bind to the active site in an extended conformation in a manner analogous to the anti conformation of ATP.